A key consideration when designing an optical communication network is the protection of the connections that traverse the network. To provide protection to the connections, a common design technique is to provide not only a working path for communications but also a protection path for communications, the protection path providing an alternative path of communications in case the working path is disabled for any reason.
In network designs, such as a Bidirectional Line Switched Ring (BLSR), the protection paths in which communications are switched are inherent to the design. For instance, if a link between two network elements within a 2F BLSR is severed, the two network elements switch the routing of communication traffic around the severed link. This is done by directing communication traffic that is to traverse the severed link on a protection path in the opposite direction to the severed link, the protection path coupling the two network elements via the other network elements within the BLSR.
The length of time in which network elements require to switch communication traffic around a fault within a network is important to ensure that integrity of the communications is maintained. For instance, according to a Bellcore standard for BLSR, the time for rerouting traffic via the protection path within a BLSR design is specified as 50 ms (after a 10 ms detection time) for a BLSR including 16 nodes with a 1200 km circumference.
One consideration with BLSR designs is how to provide sufficient protection in adequate time in cases that the network includes a large number of network elements over large distances. One technique that has been used is to subdivide the network into a plurality of BLSRs with separate inherent protection, each of the BLSRs being referred to as a protection domain within the network. To couple the protection domains together, each of the BLSRs have at least one network element coupled to a network element within another of the BLSRs. These two network elements coupled together is commonly referred to as a gateway. In many designs, the gateway consists of a single network element that is coupled within more than one protection domain.
FIG. 1 illustrates a network in which first and second BLSRs 50,52 are coupled together by a gateway 54 that consists of a first network element 56 within the first BLSR 50 and a second network element 58 within the second BLSR 52. In this network, if a fault occurs within the first BLSR 50, the protection within the first BLSR 50 route traffic around the fault while the network elements within the second BLSR 52 do not even have to be aware that a fault occurred. The opposite occurs if a fault occurs in the second BLSR 52. Therefore, if a fault occurs in either of the first or second BLSR, the time to route traffic around the fault is generally no more than 50 ms using current standards. The one exception to this time constraint is if there is a fault within the gateway 54. In this case, communications between the two protection domains could be lost. In current designs, this loss of communication is protected against by implementing match nodes such as the network elements 60,62 within the BLSRs 50,52 of FIG. 1. Match nodes are defined by Bellcore standard GR1230 and should be understood by one skilled in the art.
Within mesh networks, there is no inherent protection for a connection. For each connection within a mesh network, an Optical Connection Controller (OCC) coupled to a network element communicates with other OCCs corresponding to other network elements and establish working and protection paths for the connection within the network. These working and protection paths are unique to the particular connection and are dynamically established whenever a new connection is required within the network.
Mesh networks, as presently designed, do not have a limit on the number of network elements and links that couple together the network elements. One problem with this is that a connection between two network elements within a mesh network could be separated by a significant distance such that a connection between the network elements could traverse large numbers of network elements and large lengths of links. The more network elements and lengths of links that a connection traverses, the more likely a fault could occur in the working path for the connection. Further, the more network elements and lengths of links that a connection traverses, the longer the time that is required to switch the traffic from the working path to a protection path in the case that a fault occurs in the working path. Therefore, because of the variables involved in designing a mesh network, it is difficult to guarantee a limit on the restoration time required to reroute traffic around a fault.